High vacuum pumping method, apparatus, and techniques



April 28, 1953 E. A. HERTzLz-:R 2,636,664

HIGH VACUUM PUMPING METHOD, APPARATUS, AND TECHNIQUES Filed Jan. 28, 1949 3 Sheets-Sheet l W @ufl-vl, g'mrq April 28, 1953 E. A. HERTzLER HIGH VACUUM PUMPING IIETHOD, APPARATUS, AND TECHNIQUES Filed Jan. 28, 1949 3 Sheets-Sheet 2 Plil 28 1953 E. A. HERTZVLER 2,636,664

HIGH VACUUM PUMPING METHD, AAPP'RATUS, AND TECHNIQUES Filed Jan. 28, 1949 3 Sheets-Sheet 5 VACUUM PUMP IN V EN TOR.

LME/a' Afro/v Hf/eTzLE/e By HIS ATTORNEYS.

Patented Apr. 28, 1953 UNITED STAES PATENT O'FFICE HIGH VACUUM PUMPING ll/IETHOD,` APPARATUS, AND TECHNlQUES i Elmer Afton `Hertzler, Brooklyn,'N. Y.

Application January 2S, 1949,1SerialNoi73i318 13 Claims. (Cl. Zitti-1) The `present invention relates to pumping methods and apparatusand more Vparticularly to newand .improved pumping methods and ap paratus that are of exceptional utility at pressures approaching absolute vacuum.

At the present time, both mechanical and diffusion Apumps are used for evacuating vessels .to 10W pressures,A i. epressures approaching absolute vacuum.v The degree of vacuum obtainable withthe.formeifranges from i -2 to lll-4 mm. mercury, and pressures as low as lil7 mercury can .be obtained with the latter. In commercial operations such as the manufacture of radio tubes, for example, it is customary to use diiusion `punipsto obtain a relatively high degree-of vacuurnand theny to employ so-called g'etters for .the purpose -of making residual gases inactive. However, it has not been possible, with these devices, to secure a vacuum greaterthan 1G-8 mrn. mercury, even under the best conditions.

It isanobject oi the invention, accordingly, to 4provide new and .improved methods and apparatus for effectively evacuating a vessel to a muchlower pressure .than-has been .possible heretofore.

Another vobject of the invention is to .provide new and iinproved pumping apparatus which is capable of actingv separately and independently onindividual-molecules of a gas in a vessel.

According ,toone featureor the invention, the individual molecules oi a gas are subjected .to forces-Which cause them to move in a predetermined direction. The direction oi travel may bel for `the purpose of vcirculating the gas molecules or for urging relatively widely separated molecules away from a vessel to be evacuated. More specifically, gas molecules inthe vessel are ionizedinany suitable .manner as by collision with electrons, for example, positive and/or negativeions vbeing formed in the process. These charged ions are subjected to a potential gradient whichiorces them to move in a desired direction which` maybe .out ofv the vessehior example. In vthis fashion, .positive vor .negative ions may y bedirected t suitable .baite means where they 2 moleoulesbeingstruck is increased, and any tendencyvoifthe ions to follow random paths is reduced.

Obviously, either positive or negative ions can be removed by assigning the proper polarityto the vpotential gradient .to which the gas mole'- cules are subjected. Also, ,both kinds of ions can be removed simultaneousl-yby establishing inithe .device two potential gradients or" opposite polarites, as will'be. described hereinafter. The invention `also provides means for removing any uncharged 'or neutral particles that may be present.

Acoordingto another feature of the invention, a'vessel is evacuated to .a very high degree 0i vacuumby utilizing the method outlined briefly above .to produce a concentration of gas ions at a predetermined `location, and then removing the `concentrated gas ions by .means of new and improved getter techniques.

it will be apparent',v therefore, that the invention enables avessel to Vbe evacuated to a considerably higher degree of vacuum than has been 'possible heretofore. By ionizing theY gas molecules and utilizing a potential gradient, with or without a magnetic eld, to' force the gas ions to inove'in adesired direction, more of such residual molecules 'can be removed than is possible with'any' prior apparatus. Asa result, a degree ovacuum oan'be secured which is of the orderof` one thousand or more times the best degree `of evacuation 'hitherto thought to beobtainable.

Additional objects and advantages of the in'- vention will'be apparent from 'theollowing detailed description -of several typical embodiments, taken in V.conjunction `with the accompanying drawings,"in which:

Fig. 1 is a schematic 'diagrampartly in section, ofv pumping'apparatus constructed according to the invention Fig.r 2 is an enlarged fragmentary view, in perspective, of a portion of the apparatus shown in Fig; 1;-A

Fig,3 Iis a fragmentaryview, also in perspective, of a modified Vform ot the invention;

Fig.. e 'is a fragmentary lperspective view of another.y modification;

-Eig. 5 is a schematic diagram of another ernbodirnent designed to act upon both positive and negative particles in .avessel;.

Fig. 6 illustrates schematically a typical wayv E Y form of evacuating technique embodying the invention; and

Fig. 8 is a schematic showing of another modication of the technique illustrated in Fig. 7.

Referring to Fig. 1, a chamber Il, which is to be evacuated, is connected through an enclosure l2, which may be cylindrical in shape, for example, to a conventional vacuum pump I3. The latter may be of any desired and well known form of construction and it need not be described in detail herein.

The pump i3 is used to evacuate lthe chamber I l to the degree obtainable with pumps of known and conventional construction, which may be, say, l-8 min. mercury, for example. After evacuation in this manner, a certain number of gas molecules will still remain within the closed system comprising the chamber li and cylindrical pump enclosure l2. These molecules move in the ordinary random manner of gas molecules and if a few of them should be drawn on? by the pump i3, the effect on the remaining molecules will be relatively slight.

In order to concentrate the remaining molecules and to urge them toward the pump I3, the molecules may be ionized by collision with electrons provided by an electrical conductor It which may extend longitudinally within the cylindrical enclosure l2 substantially along the axis thereof. rlhe conductor iii is shown supported by the lead wires I5 and l5 which extend to the exterior of the enclosure i2. The enclosure I2 may be made of glass, for example, in which event the lead wires I5 and I5 should preferably be made of a material having an appropriate thermal ccefcient of expansion. The conductor ill may be heated by any suitable source of electrical energy such as a battery il, for example, through a reversing switch il', its temperature being controllable in any appropriate manner as by a variable resistor i3 included in the circuit of the lead wire l5. rilhe lead wire I6 is shown grounded at I9. The surface of the conductor Iii may be treated in any suitable manner to render it highly emissive of electrons.

Ions thus produced in the chamber II and/o1' the cylindrical pump enclosure I2 are caused to move in a desired direction by subjecting them to an electric eld of suitable` potential gradient to effect this result. The electric neld may be produced in any desired manner, as for example,

ality of annular anodes comprising end anodes 2e `and 2l and intermediate anodes 22 in spaced relationship along the interior of the cylinder pump enclosure I2, substantially concentrically both with respect to the conductor lil and to the enclosure I2. Extending from each of the annular anodes 20, ZI and 22 is a lead wire 23, each pair of adjacent lead wires 23 being shown connected together through a resistor 2li.

The anode 2l is shown connected by a further lead wire 25 through a suitable reversing switch 25 to .the positive terminal of a suitable source of anode potential indicated diagrammatically as a battery 2E, the negative terminal of which is shown grounded at 2l. The other end anode '2e is shown connected by a lead wire 28 through an adjustable resistor 29 to a three-position switch 30.

At one end of pump enclosure I2 is shown a bafe 3l (Figs. 1 and 2) which may comprise, for example a disc of ion permeable material 32 interposed between two grid-shaped electrodes 33 and 34 from which lead wires 35 and 36, re-

spectively, extend to the exterior of pump enclosure l2. The ion permeable material 32 may be any suitable porous material capable of withstanding the potential difference between the electrodes 33 and 3i, such as glass wool, for example. The grid .'-Bl is shown maintained negative with respect to ground by a suitable source of potential indicated as a battery 3l, and the grid 3E is maintained at a potential more negative than the grid 34 by means indicated diagrammatically as a further battery 38, suitable reversing switches 31 and 33 being provided to facilitate the reversal of the potentials applied to the grids 3d and S3, respectively.

In operation, let it be assumed that the switch 25 is positioned as shown in Fig. 1, so that a positive potential is impressed upon the conductor 25 with respect to ground; the switch B is in the neutral position as shown; and the switches 3l and 3S are in the positions shown so that the grid 33 is made more negative than the grid 3d, while the latter is also negative with respect to ground. Under these conditions, electrons emitted by the lament lll are attracted to the several anodes 20, 2i and 22 by reason of the positive potential applied thereto by the source 26. In the case of the intermediate anodes 22 and the end anode 2s?, the resulting current now reduces the magnitude of the positive potential by reason of the cumulative potential drops in the several resistors 2t. This renders the end anode 2] negative with respect to the other end anode 2l the intermediate anodes 22 being negative with respect to end anode 2l in varying degrees depending upon the number of intervening resistors 24. This establishes a potential gradient within cylindrical enclosure I2 such that the left end is appreciably more negative than the right end.

The switch il should preferably be positioned as shown in Fig. 1, so that the left end of conductor lll is connected to the negative terminal of source Il. When so connected, if the conductor Hl is heated from a unidirectional current source, the potential gradient along its length will be in the same direction as that established by the anodes, although not necessarily of the same magnitude.

The sources of potential difference 26, 3l and 38 should preferably be variable (adjustable) because the ionization potential or potentials of the gases within the chamber and within the cylindrical pump enclosure I2 will be quite diierent for dierent pressures of the gases. By way of example, the several potentials supplied by the sources 25, 3l and 38, respectively, might be continuously and independently variable (adjustable) from zero to say 100,000 volts. These values are not critical but can vbe varied depending upon the operating conditions desired.

Some of the electrons which are emitted from the conductor Ill strike gas molecules in traveling toward the anodes. These collisions liberate electrons from the gas molecules causing them to become positively charged ions. These positive ions are acted upon by the potential gradient within pump enclosure l2 and are urged toward the left end thereof, away from the chamber II which is to be evacuated. lt will be apparent that it is not necessary for the molecules to be present in sufficient numbers to build up any gas pressure as in pumps hitherto known. It will suffice if the structure is so arranged that the probability of every molecule within the pump enclosure i2 being struck by an electron and tliereby becomirigiionizedis relatively jh'igh. soon :as the gas fis ionized, each positive sgasiion, will .be turned llettvvar(ily 'toward :the outletfend ci the pump.

On traveling toward ythe left, 'the p'os'tiveions will be ydrawn towards the baieai rby reasonfof the additional .negative v.potential applied A.to felece trede i3d. Upon reaching the ion permeablema'ss the ions will befdrawn towards'the electrode t3 .to which afurther.negativepotentialiis applied by .t 4battery 38. The ions will thus sbe'drawn through the :nass`32 with .their freedom of:movement restricted and their velocity retarded so that they may no 'longer recirculatewithin the purnp'enclosure l2. fifter'traversingtheniass32 theyinay be deionizedfandiorm neutral gasmoleculos, and these :gas molecules .may concentrate to a density :sufficient .to zbe 'acted'iuponby the conventional vacuum pump i 3.

Under Athe conditions described, v'the xlvalues -oi' the several resistors 2d may be proportioned to obtain the desired pctentialzgradient inthepresence of current riiovv'reinilting from theattraction of electrons bythe anodes 2li, 2| and A22.` sIf desired, however, .the switch-3d may be Ypositioned to the lrigllt. so that it lrnakes contact with the contact This establishes 'a :circuit from'the source .2t through the conductorlii, the resistors 2e `and the adjustable-resistor ilitc'ground. "The resistor lil may then be so adjustedthat potential gradient `will be 'obtained evenin'lthefconiplete absence foi current .flow resulting.from the attraction 'of electrons vby the several anodes. an .et engreatcr negative potential' may be applied to the anodeld `b3,1'th1ovvln'g the switchfllt tothe left, thereby establishing 'Contact `with the contact Il@ which is connected'to the negative-terminal oi' thesource 3l.

Fig. shows a modified Storm .of the invention utilizing ran.electromagnetic `held-which is preferably directed longitudinally "of the Aconductor Hl. rihis heldin Ycombination with the electric iield'tends Vto causethe electrons emitted by 'the conductor I4 to travelinsubstantially-linearpaths towards the yright,'Whereas the positive-ions tend to movein similar llnearpaths extending'towards the left. The former condition 4increases the probability of yany 'gas `molecule being struck. Similarly, thelatter condition reduces lany tend ency of positive ions to 'follow random'paths.

The magnetic cld may be obtained in any lsuitable manner as by positioning one or more .coils #il L(Fig,3) intermediate adjacent anodes, for example. lhe coil 4l is shown diagrammatically as comprising a relatively'few turns. In practice, however, it may consist oi' a relatively large number of turns of iine resistance Wire, thus serving' both as a ycoil and as one of the resistors 2d. The coil should preferably be built up of materials `whose properties do not tend to cause adherence orfretention of gas molecules.

VIn `Fig. e is shown a further modification in which the `anodes arereplace'd by an electrically conductive `coating t2 helically `applied to the inner surface of the pump .enclosure l2. coating may have electrically resistive properties which `result vin .the production of the vdesired potential gradient andit may consistof a large number of spaced turnsfoiming the .helix,.there by producing a .relaitvely strong longitudinal magnetic iield. The lilarnent I4 and the various lead Wires vhave been ornitted'from Figs..3 and 4 for4 clarity .of illustration.

.Preferablythe pump comprising the present invention should be .kept hotinfoperation vso that The 6 any (all). fgas :molecules touching any .surface within the :pump will "be :given ysudlcient energy that Ysuch `:gas:molecules cannot become occluded uponany'surface withinthe pump,zincluding any (all) inneiwsurfacesof the .baffle 3.1. -Thepurnp apparatus may be kept `hot in .any 'desired manner :as `by electric .induction Iheaters 5.0 (Fig. 1)

for example. Also, :the conductive .coating 42*- to the-grids 33 and .Stof tliebaiiie :3L Further..

by` alternatelyV applying positive and negative potentialsgfor successive periodso'i time, :bothtposl tive `and negative iionsmayrbe removed by means of theapparatus shown in Figzl.

It yisalso :possible to remove both vpositive and negative ions .simultaneously :by connecting -two pump enclosures :l2 and ylila, 4designed Lto remove positive and vnegative ions, respectively, from-.a chamber .il to be -evacuated,.fas.shown in`Fig.:5.

In this igurefthe elements associated with vthel enclosure i 2 are substantially identical .withthose shown inlig. i and'aredesignatedbylikereference characters. :The elements associated with the enclosure 12a Aare alsoxsubstantially :identical with those .of Fig. l, and .like parts .have .been designated Aby like :reference lcharacters .with vthe subscript u. The manner of Aoperation `:of ,this niodication will be :readily apparent vfrom :the

operative description of Fig. l, which is .given above` If any electrically neutral particles are formed in .the chamber il, :in addition .to positive and negative ions, such .neutral particlescmay be removed by .introducing 'a positive ion forming Igas or :vapor suchasinercury, for example, into vthe chamberil from a conduit d '.(Fig. 5). Ionniation of this gas in the enclosures l2 and/or `lila produces positive ions Awhich move leitwardly through each/either of the enclosures 42 and/ or Hic and `through the chamber ll in Fig. 5 and carry along with them any neutral particles that may be present.

.Neutralgparticles.oarralso'beremovedibyintroducing a negative ion producing fluid into the chamber Il througlia conduit 4|.. Upon ionization Vof suchiiuidfineach/ either of theenclosures IZ'and/orl 2a (Fig. 5) negative ions move towards the right andcarry With them neutral particles that may .existin each/either-.of the enclosures t2 and/or 12a and vvin the chamber H. Obviously, neutral partici'esmay be vremoved by simultaneously introducing both positive ion forming 'and negative ion 'producing uids .through the conduits 49" Sandri 1.,;respectively, fas shown.

L'lhe. "novel vevacuating procedures described above may be combined vwith new vand improved gettertechniuues,` asfs'hown 1in Fig. 6, to achieve adegree bf vacuum'lhig'her than'is possible lwith existingxnethods. vInlig'is shown a'vessel 42 Whi'ch'is to `be evacuated to a high degree of vacuum. The-vessel v42 may be, for example, Vthe glass fenvel'op'e fof an electron 'tube having Yone armure electrodes 43,'144 and .45 .provided with leads .'46, 441 rand `v48, respectively, extending through 'the wall offthe 4'chamber '4'2 y'to the vexterior'thereor.

According to the invention, an electrode 49 is placed over a portion of the outer wall of the chamber 42 corresponding approximately to the area over which the getter material is to be deposited. The electrode 49 is connected by a conductor 50 to the negative terminal of a suitable source of high voltage D. C., the positive terminal of which is connected to the leads 46, 4l and 48 of the electrodes 43, it and 45, respectively. Also mounted within the chamber d2 and connected by a conductor 52 to the positive terminal of the source 5| is a getter holder 58. The latter is so mounted that when the getter is flashed, the getter material will be deposited upon the inner wall of the chamber i2 directly under the electrode 49.

In operation, the chamber 42 is first evacuated to a relatively high degree of vacuum by any known pumping means connected to the exhaust tube 54. A D. C. potential of several thousand volts negative (6,000 volts has given very satisfactory results in receiving size electron tubes) is then applied to the electrode i6 by the source 5|. This causes a strong'negative charge to be induced on the inside wall of the chamber d2 beneath the electrode 9. Meanwhile, the electrodes d3, lill and d5, and the wall of the chamber 42 (except where the getter deposit is formed) are being heated by suitable means such as, for

example, an electric induction heater or Bunsen burner, to drive ol and to ionize occluded gases. The positive gas ions thus formed are influenced by the electric field existing in the chamber 52 and move to the region of the inner wall thereoic directly under the electrode 59. The getter is then ashed in the usual manner. Since the getter particles are positively charged, they also tend to move toward the wall area under the electrode 49, under the influence of the electric eld.

Since the unwanted gas molecules in the chamber 42 are also concentrated at the region of the inner wall beneath the electrode 139 when the flashed getter particles arrive there, a thorough mixing of the getter and the gas takes place. In addition, the concentrationo gas molecules is trapped and held between the getter deposit 53 and the inner wall of the chamber d2. This results in a much higher degree of vacuum than is obtainable with the standard methods now employed. Furthermore, the undesired gas is held more rmly by the getter than it is when the gas is merely occluded upon the inner surface' of the getter deposit 53, as in the usual practice.

Obviously, the same results can be achieved by mounting the electrode 49 on the inside wall of the chamber ft2, a connection being made therefrom to an external terminal through the Wall of the chamber 62. In this case, the gas and the getter will both be deposited upon the electrode 49.

In the modiiication shown in Fig. 7, the exhaust tube 5A', is made of alternate sections of glass tubing 55 and copper tubing 55. In this embodiment, the vacuum within the chamber i2 is reduced to the lowest possible degree obtainable with a standard vacuum pump, whereupon the copper tubing 5B is pinched at 5l to close and seal off the exhaust tube 54. As before, the chamber 42 and the elements contained therein are heated to drive oil?, and to ionize occluded gas. the exhaust tube 54 and the negative terminal of the D. C. source 5| connected to the pinched oi segment 56 of copper tubing, thegetteris With the getter holder 58 aimed towardsV flashed in the usual manner. As a result of this action, the unwanted gas molecules are trapped on the inside wall of the lowermost segment 56 of copper tubing, where the flashed getter material is deposited. By pinching the next segment of copper tubing 56 directly above the initial pinch, these trapped molecules can be removed.

I, after operation for a period of time, some gas molecules leak into the chamber 42, the getter can be flashed again and the next segment 56 of copper tubing pinched to remove trapped gas molecules and restore the vacuum to its original high condition.

As a further modification, additional getter holders 58a and 58h may be placed at different positions within the exhaust tube 54 where they can be dashed at dilerent subsequent times to restore the desired vacuum conditions to the.

chamber l2 after periods of use, as shown in Fig. 8.

From the foregoing, it will be apparent that` the invention provides highly effective methods and apparatus for evacuating a vessel to a high degree of vacuum i. e., a vacuum approaching absolute vacuum. By ionizing gas molecules within the .chamber and influencing the ions thus produced with an electric eld, the ions may be caused to move in a predetermined direction to a collection point in order that they may be removed. Further, by combining this vacuum pumping procedure with the improved getter techniques described above, degrees of vaccum can be secured that are far higher than any obtainabie with prior methods and apparatus.

It will be understood that the several representative embodiments described are susceptible of numerous modifications in form and detail within the spirit of the invention. For example, whereas only a single baille 3| is shownin Fig. l, in practice, one or more of such baffles might be employed. Also, although the getter techniques illustrated in Figs. 6-8, inclusive, are

directed to the inactivation of positive ions, it.

will be understood that the same techniques with the appropriate polarities, would be applicable to the inactivation of negative ions. Other variations will occur to those skilled in the art. The

specific embodiments described, therefore, are not to be regarded as limiting the scope of the following claims.

I claim:

l. In a device of the class described, means for causing the ionization of gas molecules, means for ized molecul-es for retaining the ionized moleculesl after movement in the pred termined direction for a predetermined distance.

3. In a device of the class described, an enclosure, means arranged to ionize gas molecules within the enclosure, electrostatic means for producing a potential gradient within the enclosure for urging the ionized molecules in a predetermined direction and nonconducting, ion permeable.

9 means disposed in the path of said ionized molecules for retaining the ionized molecules after movement in the predetermined direction for a predetermined distance.

4. In a device o the class described, an enclosure, electron emitter within the enclosure, anode means for accelerating electrons from the emitter 'to cause ionization of gas molecules within the enclosure upon collision with said electrons, means for urging the ionized molecules in a predetermined direction within the enclosure, and nonconducting, ion permeable means disposed in the path of said ionized molecules for retaining the ionized molecules after movement in the predetermined direction ior a predetermined distance.

5. In a device of the class described, the combination of a vessel to be evacuated, a pair oi enclosures connected tc said vessel, means for ionizing gas molecules in said enclosures, means in one of said enclosures 4for urging positive ions away from said vessel, nonconducting, ion permeable means disposed in the path of said positive ions for retaining molecules formed therefrom, means in the other of said enclosures for urging negative ions away from said vessel, and nonconducting, ion permeable means disposed in 'the path of said negative ions for retaining molecules formed therefrom.

5. In a device of the class described, the combination of a vessel to be evacuated, an enclosure connected to said vessel, means for ionizing gas molecules in said enclosure, means for urging ions in said enclosure away from said vessel, noncomducting, ion permeable means disposed in the path of said ions for retaining molecules formed therefrom, and means for introducing an ionizable fluid into said vessel,

7. In a device of the class described, the combination of a vessel to be evacuated, a pair of enclosures connected to said vessel, means for ionizing gas molecules in said enclosures, means in one of said enclosures for urging positive ions away from said vessel, nonconducting, ion permeable means disposed in the path of said positive ions for retaining molecules formed therefrom, means in the other of said enclosures for urging negative ions away from said vessel, nonconducting, ion permeable means disposed in the path of said negative ions for retaining molecules formed therefrom, and means for introducing both positive and negative ion forming fluids into said vessel.

8. In a method of circulating gas molecules, the steps of ionizing said gas molecules, iniluencing said ionized molecules with an electric field to cause ions to move in a predetermined direction to a reception position, and impeding the movement of said ions at said reception position by directing them to a nonconducting, ion permeable baille member to produce a concentration of ions thereat.

9. In a method of evacuating a vessel containing gas, the steps of pumping said gas out of the vessel until only relatively few gas molecules remain, ionizing said gas molecules, creating an electric charge on a portion of the inner wall of said vessel, thereby causing ions to collect in the vicinity of said charge, disposing a getter in said vessel, and :dashing said getter on said charged portion so as to trap the ions between the flashed getter and said charged portion of the wall of the vessel.

l0. In a method of evacuating a vessel having an exhaust tube provided with a metallic portion and containing gas, the steps of pumping out said gas until only relatively few gas molecules remain, ionizing said gas molecules, establishing an electric charge on said exhaust tube metallic portion, thereby causing gas ions to collect thereat, flashing a getter over said collected ions, and pinching off said exhaust tube in the vicinity of said metallic portion to remove said collected gas ions.

l1. In a device for moving gas molecules, the combination of an enclosure, an elongated electron emitter in the enclosure, a plurality of tubular electrodes mounted coaxially of said emitter and spaced apart therealong, resistance means electrically connected between adjacent tubular electrodes, a source of electrical energy connected to the emitter and to at least one of said tubular electrodes, a pair of longitudinally spaced apart screen electrodes mounted in said enclosure near one end of said emitter, a body of ion permeable material disposed between said screen electrodes, and means for controlling the potentials of said screen electrodes with respect to a reference potential.

12. A device as deilned in claim 11 in which the resistance means electrically connected between adjacent tubular electrodes comprises a magnetizing winding disposed coaxially with respect to the electron emitter so as to establish a magnetic field ccaxially of the emitter.

13. In a device of the class described, an enclosure, an electron emitter within the enclosure, anode means for accelerating electrons from the emitter to cause ionization of gas molecules Within the enclosure upon collision with said electrons and electrical means connected to said anode means to establish a potential gradient within the enclosure for urging the ionized molecules in a predetermined direction, said anode means comprising helical coil means wound about an axis extending in said predetermined direction.

ELMER AFTON HER'IZLER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,950,008 Mutscheller Mar. 6, 1934 2,022,465 Hansell Nov. 26, 1935 2,160,863 Hickman June 6, 1939 2,246,327 Slepian June 17, 1941 2,282,401 Hansell May 12, 1942 2,404,997 Tills et al. July 30, 1946 2,460,175 Hergenrother Jan. 25, 1949 2,469,006 Shelby May 3, 1949 

